Proffitt et al (2003) reported that increases in perceived effort (by wearing a heavy backpack) result in overestimation of egocentric distance. If true, this has important implications for theories of visual space perception. Here, we attempted to verify this finding, first by replicating the methods closely and then by using a well-established rating scale to directly evaluate whether wearing a backpack indeed influences perceived anticipated effort.

Participants verbally estimated egocentric target distances while either wearing a backpack weighted 1/5 to 1/6 of their total body weight (n=12, 6 females) or without wearing a backpack (n=12, 6 females). Targets were randomly presented in one of six directions on the ground to prevent the use of landmarks as distance cues. Participants were given two blocks of six practice trials and two blocks of six test trials. Using similar methods, other participants rated how much effort would be required to walk to various targets, using the Borg CR-10 perceived effort scale (backpack: n=11, 6 females; no backpack: n=12, 6 females). Test distances for both studies ranged from 4 to 14 m.

We found no effect of backpack on verbal estimates of egocentric distance (F=.05, p=.82).

Our results contrast sharply with those of Proffitt et al (2003). We followed the previously-reported methodology closely, so our results suggest that the effect is sensitive to as-yet undetermined methodological factors. One possible interpretation is that effects of perceived effort on distance perception do occur, but that they occur under a more limited set of circumstances than has previously been supposed. Another possibility is that some factor other than changes in perceived distance influences the calibration of verbal reports when observers don a heavy backpack.